As technology nodes decrease in size, the density of features on a substrate increases. The increased feature density requires more precise control of spacing between features to produce viable devices. Feature spacing is determined in part by the feature layout on the reticle used in patterning the substrate. Additional factors such as overlay error also impact the spacing of features on the substrate. Overlay error is the difference between the feature layout on the reticle and the actual feature pattern formed on the substrate.
Overlay error is caused by correctable factors such as differences in thermal expansion coefficients between the reticle and the substrate, inaccuracy in substrate shape, misalignment of the reticle and the substrate, inaccurate calibration of actuators in the patterning apparatus, etc. Differences in thermal expansion cause overlay error because the patterning process generates thermal energy which causes the substrate and the reticle to expand. If the materials used to form the substrate and reticle have different thermal expansion coefficients, the rate of expansion for the substrate and reticle are different. The different rates of expansion result in different feature spacing on the reticle as compared to the spacing of features on the substrate. Inaccuracy in substrate shape causes overlay error because curves in the substrate surface cause the dimensions of a feature to change because patterning light travels a different distance to reach the substrate surface. Misalignment of the reticle and substrate causes overlay error because the entire layout is shifted. Inaccurate calibration of actuators causes overlay error because patterning processes use actuators to position the reticle, the substrate and other components of the patterning apparatus. If the movement caused by the actuators is not accurate, the spacing between features is changed by the difference in the intended movement from the actual movement.
Additionally, some overlay error is caused by uncorrectable factors, such as light scattering or thickness gradients in a photoresist layer on the substrate. This uncorrectable overlay error is called residual error. Some amount of residual error is unavoidable.
Some techniques measure the overlay error by using fixed, predetermined reference points. The reference points are selected prior to performing any patterning process and fail to account for actual measured differences between the reticle and the substrate. The position of several selected features on the reticle is measured, prior to patterning the substrate. The substrate is then patterned and the position of these same selected features is measured on the patterned substrate. The difference between the position of the selected features on the reticle and the position of the selected features on the substrate determines the overlay error.
A measurement system for measuring the position of features on a substrate includes a radiation source, a detector and a stage to support the substrate. Light from the radiation source contacts the patterned surface of the substrate and is reflected toward the detector. The light from the radiation source is targeted on the predetermined reference points. The detector transfers the information to a calculating unit, such as a computer, to determine the position of the selected features on the substrate.